The present invention relates to a color image forming apparatus such as an electrophotographic copier, printer or the like and, in particular, to an apparatus and method for stabilizing the charge-to-mass ratio of the various toner components in such a color imaging system.
Electrophotographic imaging (or xerography) is a well known method of copying or otherwise printing documents. In general, electrophotographic imaging uses a charge-retentive, photosensitive surface (known as a photoreceptor) that is initially charged uniformly. The photoreceptor is then exposed to a light image representation of a desired image that discharges specific areas of the photoreceptor surface creating a latent image. Dry toner powder is applied to the latent image, forming a developed image. This developed image is then transferred from the photoreceptor to a substrate (e.g. paper, transparency, and the like) generally by the use of electrostatic attraction between the charged toner particles and oppositely charged ions sprayed on the substrate by a transfer charger.
This electrophotographic process can be used to produce color images by repeating the above-described process for each color of toner used. However, in this tone-on-tone (also known as xe2x80x9cimage-on-imagexe2x80x9d) color accumulation approach, recharging the photoreceptor between toner applications results in increasing the charge-to-mass ratio of any toner previously deposited on the photoreceptor until the toner reaches a saturated charge-to-mass ratio. Unfortunately, the last toner color applied is not subjected to any recharging; consequently it has a different charge-to-mass ratio than the other toner colors. Inasmuch as the charge-to-mass ratio of toner affects its transfer efficiency, image quality is likely to be adversely affected by this disparity in the charge-to-mass ratios.
While various prior art references disclose the use of a pretransfer charging corona to ensure that all of the toner particles have the same electrostatic polarity, none of those references acknowledge the problem of differing charge-to-mass ratios between toner particles of the same polarity. Still other electrophotographic processes including: Rees, U.S. Pat. Nos. 5,828,933 and 5,978,628 and Appel, U.S. Pat. No. 5,933,182, teach the use of a pretransfer erase lamp in addition to a corona charger to encourage uniform charging of the toner components. This approach adds additional cost and results in a lower overall charge-to-mass ratio, thus, requiring the generation of a higher electrostatic attraction to transfer the toner from photoreceptor to substrate.
In yet another prior art approach to tone-on-tone electrophotography a selective pretransfer charger for use in association with a tri-level, highlight electrophotography system that uses two wavelengths of light to discharge a uniformly charged photoreceptor surface to three electric charge levels (each level corresponding to the latent image of one toner color) is used. See Parker, U.S. Pat. No. 5,895,738. This electrophotographic approach is highly complex.
Thus, among other potential needs, a need exists for a method and apparatus for tone-on-tone electrophotography having toners of a higher, substantially uniform pretransfer charge-to-mass ratio.
The present invention relates to a method and apparatus for efficiently transferring at least two subtractive color toner images simultaneously onto a substrate. The apparatus includes a photoreceptor (drum or belt), a transfer charger operably associated with the photoreceptor; a mechanism for placing the substrate between the photoreceptor and transfer charger; and a plurality of print station. Each of the print stations is operably associated with the photoreceptor to form a plurality of color images thereon in registration with one another to form a subtractive color image on the photoreceptor. Each print stations includes a charger unit, an exposure unit, and a developer unit for applying a respective color toner to the photoreceptor, while these print stations are preferably uniform to aid in maintenance of the unit.
Upon application of a color tone image to the photoreceptor each color images consists of toner having an initial charge-to-mass ratio. Essentially, an example is shown wherein each of charger units charges the photoreceptor to substantially the same potential and induces a saturated charge-to-mass ratio in any toner previously laid down on the photoreceptor. This saturated charge-to-mass ratio being significantly greater than said initial charge-to-mass ratio.
Consequently, the apparatus further includes a stabilizing charger unit, which substantially conforms the charge-to-mass ratio of the last applied toner to the previously applied toner before the composite color images are electrostaticlly attracted from the photoreceptor to the first substrate surface by operation of the transfer charger. In this manner, the apparatus ensures transfer rate uniformity, thus, leading to potentially improved image quality. The stabilizing charger unit includes a non-contact charging system, which may include a corona wire and may even be a corotron, scorotron and pin scorotron. In one example, the print station charger and stabilizing charger units are the same apparatus type.
The method for efficiently transferring a composite toner image having at least two subtractive color toner images simultaneously onto a substrate from a photoreceptor includes: (a) charging the photoreceptor to a desired potential; (b) forming a respective color latent image on the photoreceptor; (c) developing the respective color latent image with a respective color toner to form a respective color toner image with the respective color toner having an initial charge-to-mass ratio; (d) repeating steps (a) through (c) for each of the color toner images, including a last color toner image, that together completely form the composite toner image; (e) charging the color toner of the last toner image to the saturated charge-to-mass ratio; and (f) electrostatically transferring the composite toner image onto the substrate surface.